GDScript: Allow empty parentheses for property getter declaration

[dalexeev]

• Closes Allow declaring a getter using get(): additionally to get: godot-proposals#8057.
• Closes Improve the error message when declaring getter with get(): instead of get: godot-proposals#8058.

[Calinou]

Tested locally, it works as expected.

Syntax highlighting as it appears in the editor:

Testing code:

var hello = 25:
	get():
		print("get: hello")
		return hello
	set(value):
		print("set: hello = %d" % value)
		hello = value

func _ready() -> void:
	hello = 25
	hello = 26
	hello = 27
	print(hello)

[akien-mga]

Thanks!